Unsplash app & dashboard picture resolution

Hey you pro!

NC 21
php 7.3

It’s about the background picture in the dashboard and header. I’m using the Splash app and the pics are very blurry

I’ve tried to solve this by changing the settings in /apps/unsplash/css/login.css and header.css by inserting a high resolution unsplash url (https://source.unsplash.com/3840x2160/?nature).
But that didn’t work out.

Is there any other file I have to adjust?

Thanks for your thoughts and answers!